Posted by d on March 12, 2014, 5:41 pm
I don't follow the working in question 2 of the sample exam. For the 2nd part of the question (the 10 mark part), how do you get from
sum (e^(-ikna) - 1) to sum 2(cos(kna) - 1) ??
[n] [n>0]
(i.e. the second line of working in the solution)
Clearly the 2 is introduced due to going from a sum over all n to a sum just over positive n but I would have thought
e^(-ikna) -1 = 2cos(kna) - e^(ikna) -1
Help is much appreciated.
Thanks!
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